Problem: In the right triangle shown, $AC = 4$ and $AB = 9$. What is $BC$ ? $A$ $C$ $B$ $4$ $?$ $9$
Answer: We know $a^2 + b^2 = c^2$ We want to find $a$ ; let $b = 4$ and $c = 9$ So $a^2 = c^2 - b^2 = 9^2 - 4^2 = 65$ Then, $a = \sqrt{65}$.